It is well known that the cost of providing electricity during peak demand can be many times that of the cost of providing electricity throughout the rest of the year. These peak times can be caused by power shortages due to unexpected power plant outages, cold weather where electric heat is prevalent, congested areas where local demand has outgrown transmission capability or most commonly on hot summer afternoons when air conditioning loads are highest. In the case of summer air conditioning loads, the peak demand averages less than 50 hours and very rarely exceeds 100 hours in any given year. For most of these conditions, electric supply is supplemented by local or distributed generating equipment. The cost of this distributed generating equipment becomes very high when applied to the very few peak hours of annual operation.
Many of the load management systems designed to reduce peak demand, are oriented on the fact that most electricity grids are not designed to store electricity. Electricity must be generated when needed and used when generated. As a result most electricity management is focused on the current or momentary condition of the overall system. This perspective has lead to many reactive load management systems that simply seek to shed loads when demand is critically high. The execution of such a system involves a complicated evaluation of the customer's preferences regarding which loads should be given power and which loads should be shed, under a variety of conditions. In addition, many of these load shedding systems have a central management system directing many load controlling devices needed to execute the many programmed preferences and schedules. In general, the more sophisticated systems require more components, which create incremental increases in cost.
There are several rotational duty cycle systems on the market that cycle loads on and off during times of high demand. One example is a timing switch installed in the power line of a central air conditioning compressor, which cycles power on and off at set periods, usually every 15 minutes. These duty cycles are often staggered such that only half of the duty cycle switches in a service area are on at a given time. Essentially half of the switches will allow air conditioning on the hour and at the half hour, with the other half of the switches allowing air conditioning at 15 and 45 minutes past the hour. These rotational duty cycle systems are successful in producing an overall effect but have their effectiveness limited by the loads with timers. Other loads in the home are uncontrolled allowing them to add to the peak demands. In addition, the system creates an inequity between customers with oversized central air conditioning systems and customers with properly sized central air conditioning systems. If a central air conditioning system is sized at 200% of the cooling load, then it normally runs just 50% of the time. Therefore, holding the run time to 50% will have zero effect on the electricity consumed. Conversely, the central air conditioning system sized at 100% of the cooling load will save considerable energy given the duty cycle switch will reduce the run time to half of what it would be in normal operation. The range of inequities across these systems will vary proportionally with the extent to which the air conditioning systems are oversized.
Lastly, energy management systems to date provide a limited level of control over facilities total energy usage. The major electrical loads are configured with control devices and the energy usage for just these loads is somehow managed in a way consistent with the operator's wishes. These systems reduce the facility load but do not pursue the goal of limiting total energy usage. The uncontrolled loads in the facility will create a random effect on the total energy usage.